Electroluminescent image device



United States Patent ()ffice 2,884,541 Pat nted Apr. :28, 1959 .ELECTROLUMINESCENT. IIVIAGE DEVICE Frederick H. Nicoll, Mercer County, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application ctober.10, 1955,.Serial No. 539,519

4 Claims. (Cl. 250-213) This invention relates to electroluminescent image devices, and particularly to such devices as are designed primarily for intensifying light or light images.

This invention is directed to an improvement of electroluminescent :devices of the type which comprise a first layer of electroluminescent phosphor material, a second .layer of a material having a variable impedance characteristic in response to incident radiant energy (e.g. photoconductive material) on one surface of the electroluminescent layer, anda plurality of spaced apart'elongated conductors, such as strips, on the other surface of the electroluminescent layer. -In operation, a voltage is applied between the adjacent conductors so that with no light incident on the photoconductive layer the impedance of thephotoconductive layer is high enough to limit any effective current flow between the conductors. However,'when lightor a light image is projected onto the photoconductive layer, the impedance of the photoconductive layer is reduced in the illuminated areas by an amount corresponding to .the intensity of the incident light, so asto produce a low resistance path between adjacent :conductors and thus a corresponding increase in the current flow between the conductors in the illuminated areas and through the electroluminescent layer sufiiciently tocause the electroluminescent phosphor .to luminesce.

Thus, any variations in intensity of the projected light, as willbe present in a light image, will produce corresponding variations'in the electric current flowing through the electroluminescent layer, with resulting corresponding'variations in the electroluminescentlight.

In the operation of such a device, the electric field extending between adjacent conductors is strongest at their adjacent edges and drops off rapidly in the direction away from the edges. This same electric field extending through the electroluminescent layer and the photoconductive layer progressively weakens in the areas away from the adjacent edges of the conductors. The result is a reduced effective areaof light emission from'the electroluminescent layer and a loss in sensitivity of the photoconductive layer, dueto localization of theelectric field, such that high light gains even with increased voltage are difficult to achieve.

Accordingly, a principal object of this invention is'to improve the efiiciency'of electroluminescent devices of the type referred to.

Briefly, the invention'consists in interposing in a device of the type described a mosaic of discrete conductive elements intermediate a photoconductive layer and electroluminescent phosphor, with the conductive elements registered with spaced-apart elongated conductors, such as strips, on the other side of the phosphor. Each'conductive element and the portions of the electroluminescent phosphor and conductors registered therewith constitutes a discrete electroluminescent cell. When a potential difierence is applied between adjacent conductors, the

resultant electric field produced in adjacent electroluminescent cells is more .uniform and the actual light .emitting areais increased. Similarly,.the electric fieldappearing through the photoconductive layer between the conductive elements of adjacent electroluminescentcells is increased and thesensitivity thus improved.

In the drawing:

Fig. 1 isa partial plan view with portions removedof an electroluminescent;device-constructed according to the invention;

Fig. 2 is a partial sectional view taken'along the lines 22 of Fig. 1;

Fig. 3 is a partial sectional view showing a modification of the invention; and

Fig. 4 is a partial sectional view showing .still another modification of the invention.

In Figs. 1 and 2 there .is shown atrans parent support member 10, such as a plate of glass or plasticysupporting on a surface thereof two sets of intermeshed elongated conductors 12 and 14. The conductors 12 and 14 are transparent and may comprise tin oxide, tin chloride, or any other suitable conductive material applied as very thin strips having substantial length compared to their width. Each set of. conductors 12 and 14 has a common electrical connectiomsuch as bus bars 16 and 18 respectively for making electrical connection to a voltagesource 20.

.A layer of electroluminescent phosphor material .22'is applied over the conductors 12 and .14. The phosphor layer 22 may comprise any suitable electroluminescent phosphor-material, preferably embedded in a dielectric binding material,.such as 'ethyl cellulose, .polystyrene or an epoxy resin such as Araldite. Suitable phosphor materials are zinc sulfide activated with copper, and :zinc selenide activated with manganese, forexample. .Aiithin layer of light opaque insulating material 24is applied over the electroluminescent phosphor layer 22. The opaque .layer .24 may comprise lampblack mixed with insulating material, such as ethyl cellulose. In some cases it may be preferred'to omit the opaque layer, as -wi1-l be explained later.

A mosaic-of conductive elements 26, -each 'of'which may be rectangular, is applied on the opaque la ye'r'2'4. The 'condu'ctive elements 26 form rows registering'with the conductors 12 and 14. Eachconductive element 26 and the registering portions of the electroluminescent phosphor layer 22 and "the conductors 12-01 14 constitute an elemental electroluminescent cell. 'The conductive elements 26 may be formed from'silver paste orother metallic powder mixed in a suitable binder, for-"example.

A layer of photoconductivematerial 2 8 is applied in intimate contact with the mosaic elements 26 but 'not necessarily bonded thereto. The layer '28 may comprise any material which'has a variable impedance characteristic in response to incident radiation. It may comprise a powdered layer of cadmium sulfide'or cadmium selenide mixed with a dielectric binder such as the binders used for the electroluminescent layer 22. These and other photoconductivepowders are disclosed in copending'a'pplication of Charles J. Busanovich and Soren M. Thomsen, Serial No. 472,354, filed December 1, 1954, nowULS. Patent Number 2,876,202. When a powder layer is used, it may be applied directly by spraying over the mosaic elements 2 6. However, it is preferred to use a sintered photoconductive layer, such as is disclosed in copending application of Soren M. Thornsen, Serial No. 473j0'U 1, filed December 3, 1954, now US. Patent No. 2,765,385. The s'intered layer may comprise suitably activated eadmium sulfide or cadmium selenide, for example. Such a will assure intimate contact between the photoconductive layer 28 and the conductive elements 26 when the two plates 10 and 30 are joined together.

As an example of the physical dimensions used, the elongated conductors 12 and 14 may be strips .040 inch wide and spaced .020 inch apart. The electroluminescent layer 22 and opaque layer 24 may each be .001 inch thick. The photoconductive layer 28 may be a sintered layer of cadmium selenide of a few microns thickness. If a photoconductive powder layer is used it may be .005 inch thick. The conductive elements 26 may be rectangular, .040

inch long in the direction perpendicular to the elongated conductors 12 and 14, .020 inch wide, and spaced .020 inch apart in the direction parallel to the elongated conductors 12 and 14.

In operation, the source voltage 20 is preferably alter nating current of several hundred cycles per second he quency and of the order of five or six hundred volts. The source 20 establishes a potential difference between adjacent conductors 12 and 14. This causes an electric field to be established between the conductors. Referring to Fig. 2, consider one pair of adjacent conductors 12 and 14, and one pair of adjacent conductive elements 26, that is, a pair of conductive elements lying in adjacent rows. These two conductive elements 26, together with the registering portions of the conductors 12 and 14 and the portions of the electroluminescent layer 22 therebetween, constitute two elemental electroluminescent cells. The electric field extends through one elemental electroluminescent cell, then across that portion of the photoconductive layer 28 between the adjacent conductive elements 26, and then through the other elemental electroluminescent cell.

In the absence of light on the elemental area of the photoconductive layer 28 extending between the pair of adjacent conductive elements 26, the impedance of the photoconductive layer 28 will be substantially higher than the impedance of each of the electroluminescent cells. This may be elfected by making the thickness of the electroluminescent layer 22 of each elemental cell small as compared to the width of photoconductive material between adjacent conductive elements 26. Such relationship exists in the example cited above. Under these circumstances, the electric field appearing across each elemental electroluminescent cell, and the resulting electric current flow therethrough will be too low to cause electroluminescence.

With light incident on the elemental area of the photoconductive layer 28, the impedance of the photoconductive layer will be reduced in that area to such an extent that the increased electric field across each elemental electroluminescent cell, and the increased electric current therethrough, will be sufiicient to produce electroluminescence. Light gain is achieved by virtue of the power supplied by the voltage source. Inasmuch as the current through an electroluminescent cell is a function of the intensity of the incident light, it is possible to project a light image L on the photoconductive layer 28 and produce an amplified light image L from the electroluminescent layer 22.

The presence of the mosaic of conductive elements 26, in accordance with the invention, provides discrete elemental electroluminescent cells and direct photoconductive paths between each cell. In this way, a more uniform electric field is produced through each elemental 4 electroluminescent cell rather than concentrations of electric field appearing at the edges of the conductive strips, with the result that a greater area of light emission is obtained from the phosphor. Similarly, a more uniform electric field is produced across the photoconductive layer, with resulting improvement in photosensitivity.

The function of the opaque layer 24 is to prevent light feedback from the electroluminescent layer 22 to the photoconductive layer 28. In cases where light feedback is desired, for example for the purpose of storing a visible image after the incident image has been removed, the opaque layer 24 may be omitted and the conductive elements 26 laid down directly on the electroluminescent phosphor layer 22.

In the modification shown in Fig. 3, each conductive element 32 is provided with an embossment 34 and only the embossments 34 are in contact with the photoconductive layer 28, otherwise the structure of Fig. 3 is like that of Figs. 1 and 2, like numerals identifying like elements. The embossments 34 may be formed as ribs, or they may be bead-like. The embossments 34 may be formed by machining operations performed on the elements 32. Alternatively, the elements 32 may be machined flat and then the embossments 34 may be laid down on the machined surface by silk screening techniques, for example. The embossments 34 provide a longer photoconductive path between the conductive elements 26 and hence a greater useful area of photoconductor. This permits more eifective use of the incident light falling on the device and therefore increases its sensitivity. The operation is similar to the device of Figs. 1 and 2.

In the modification shown in Fig. 4, the electroluminescent material and opaque material are laid down in strips 36 and 38 respectively in registry with the elongated conductors 12 and 14. In this case, the conductive elements 26 are preferably formed without embossments. By means of this construction, the underside of the photoconductive layer 28 is exposed through the glass plate 30 so that an incident light image L may be projected from the opposite side shown in Figs. 1 and 2. In this way, the projected image L and the output image L; are on the same side. The opaque strips 38 may be omitted if the conductive elements 26 are made thick enough to be opaque by themselves. In this modification, light feedback can occur only inefliciently by reflection from the glass plate 10.

By means of the invention, then, electric fields of greater uniformity are produced in the electroluminescent cells and the associated photoconductive areas. The beneficial results thereby produced are increased light emission area and improved photosensitivity.

What is claimed is:

1. An electroluminescent image device comprising an array of spaced apart elongated conductors arranged side by side, adjacent ones of said conductors being electrically insulated from each other, electroluminescent phosphor material on each of said conductors on the same side of said array, 21 mosaic of mutually insulated conductive elements adjacent to said phosphor material, each of said conductive elements being registered with a conductor to form an electroluminescent cell with the phosphor therebetween, photoconductive material on said mosaic and contacting each of said conductive elements and bridging the gaps therebetween, and lead means connected to said conductors for applying a potential difference between adjacent conductors.

2. The invention according to claim 1 wherein said phosphor material comprises strips registered with said conductors.

3. An electroluminescent image device comprising a transparent support plate, two electrically insulated sets of intermeshed transparent conductive strips on said support plate, a'layer of electroluminescent phosphor material overlying said strips, a second transparent support plate hearing on a surface thereof a sintered photoconductive layer, and means maintaining said two layers in close-spaced relationship, said means including a mosaic of mutually insulated conductive elements contacting said photo-conductive layer and registered With said strips.

4. The invention according to claim 3 wherein each of said elements includes an embossment in contact with said photoconductive layer.

References (Iited in the file of this patent UNITED STATES PATENTS White Aug. 25, 1953 Weimer Oct. 6, 1953 Kalfaian Dec. 27, 1955 Cassman May 15, 1956 Kazan et a1. M Oct. 23, 1956 

